The Army microbiologist who was working on a vaccine for anthrax poisoning was now being grilled as a suspect in the 2001 letter attacks that killed five people and sickened 17. FBI agents were staking out his house.

Ivins committed suicide this week before he could be charged with murder for mailing the toxin-laced letters, which spread nationwide alarm just weeks after the Sept. 11, 2001, terror attacks in New York and Washington. Authorities said the letters, filled with anthrax powder, may have been a warped plot for Ivins to test his vaccine on victims.

Ivins' lawyer said the brilliant but troubled scientist was innocent but would never have a chance to prove it.

Friday's sudden naming of Ivins as the top — and perhaps only — suspect in the anthrax attacks marked the latest bizarre twist in a case that has confounded the FBI for nearly seven years. Last month, the Justice Department cleared Ivins' colleague, Steven Hatfill, who had been wrongly suspected in the case, and paid him $5.8 million.

Ivins worked at the Army's biological warfare labs at Fort Detrick, Md., for 18 years until his death on Tuesday. He was one of the government's leading scientists researching vaccines and cures for anthrax exposure. But he also had a long history of homicidal threats, according to papers filed last week in local court by a social worker.

Letters containing anthrax powder were sent on the heels of the terror attacks of Sept. 11, 2001, and turned up at congressional offices, newsrooms and elsewhere, leaving a deadly trail through post offices on the way. The powder killed five and sent numerous victims to hospitals.

Workers in protective garb that made them look like space men decontaminated U.S. Capitol buildings after anthrax letters were discovered there. Major postal substations were closed for years. Newsrooms were checked all over after anthrax letters were mailed to offices in Florida and New York.

The Justice Department said Friday only that "substantial progress has been made in the investigation." The statement did not identify Ivins.

However, several U.S. officials said prosecutors had been closing in on the 62-year-old Ivins and planned to seek an indictment and the death penalty. Authorities were investigating whether Ivins, who had complained about the limits of testing anthrax drugs on animals, had released the toxin to test the treatment on humans.

The officials all discussed the continuing investigation on condition of anonymity because they were not authorized to speak to the media.

The Justice Department is expected to decide within days whether to close the "Amerithrax" investigation now that its main target is dead. If the case remains open, that could indicate there still are other suspects.

Ivins' attorney asserted the scientist's innocence and said he had cooperated with investigators for more than a year.

"We are saddened by his death, and disappointed that we will not have the opportunity to defend his good name and reputation in a court of law," said Paul F. Kemp.

Colleague blames 'pressure'
Ivins died Tuesday at Frederick Memorial Hospital in Maryland. Relatives told The Associated Press that he killed himself. Kemp said his client's death was the result of the government's "relentless pressure of accusation and innuendo."

For more than a decade, Ivins had worked to develop an anthrax vaccine that was effective even in cases where different strains of anthrax were mixed — a situation that made vaccines ineffective — according to federal documents reviewed by the AP. In 2003, he shared the Decoration for Exceptional Civilian Service for his work on the anthrax vaccine. The award is the highest honor given to Defense Department civilian employees.

Ivins conducted numerous anthrax studies, including one that complained about the limited supply of monkeys available for testing. The study also said animal testing couldn't accurately show how humans would respond to anthrax treatment.

The Fort Detrick laboratory and its specialized scientists for years have been at the center of the FBI's investigation of the anthrax mailings. In late June, the government exonerated Hatfill, whose name has for years had been associated with the attacks. Then-Attorney General John Ashcroft called him a "person of interest" in 2002.

Investigators also had noticed Ivins' unusual behavior at Fort Detrick in the six months following the anthrax mailings. He conducted unauthorized testing for anthrax spores outside containment areas at the infectious disease research unit where he worked, according to an internal report. But the focus stayed on Hatfill.

Ivins' friends, colleagues and court documents paint a picture of a flourishing scientist with an emotionally unstable side. Maryland court documents show he recently received psychiatric treatment and was ordered to stay away from a woman he was accused of stalking and threatening to kill.

History of 'homicidal threats' Social worker Jean C. Duley filed handwritten court documents last week saying she was preparing to testify before a grand jury. She said Ivins would be charged with five capital murders.

"Client has a history dating to his graduate days of homicidal threats, plans and actions towards therapists," Duley said, adding that his psychiatrist had described him as homicidal and sociopathic.

Authorities have been watching Ivins for some time. His brother, Tom Ivins, said federal agents questioned the scientist about a year and a half ago. Neighbors said FBI agents in cars with tinted windows conducted surveillance on his home. A colleague, Henry S. Heine, said that over the past year, he and others on their team had testified before a federal grand jury in Washington that has been investigating the anthrax mailings.

On July 10, police responded to Fort Detrick to speak with Ivins. He was ultimately removed from his job and taken to a hospital for psychiatric evaluation because of concern he had become a danger to himself or others.

The victims of the attacks had little in common.

Robert Stevens, 63, a photo editor at the Sun, a supermarket tabloid published in Boca Raton, Fla., was the first to die.

Thomas Morris Jr. 55, and Joseph Curseen, 47, worked at a Washington-area postal facility that was a hub for sorting the capital's mail.

Kathy Nguyen, 61, who had emigrated from Vietnam and lived in the Bronx, worked in a stock room at Manhattan Eye Ear & Throat Hospital. The last to die was Ottilie Lundgren, 94, who lived in Oxford, Conn.

NBC News' Pete Williams and The Associated Press contributed to this report.